1. Field of the Invention
This invention relates to a process for producing terephthalic acid by oxidizing p-xylene with molecular oxygen.
2. Description of the Prior Art
Recently, direct polymerization of terephthalic acid and ethylene glycol has been adopted in the preparation of polyethylene terephthalate. It is required that this direct polymerization process use a relatively pure terephthalic acid having the impurity content of not more than 300 ppm as the starting material. Representative of impurities present in terephthalic acid is 4-carboxybenzaldehyde (hereinafter referred to as 4-CBA). Hence, raw terephthalic acid as prepared is usually subjected to purification procedure prior to use in the direct polymerization process.
More recently, there has been proposed a process for preparing terephthalic acid of high purity in one step without additional purification by using a highly active catalyst. This process is attractive from an economic standpoint since it can eliminate the purification step, thus saving greatly the production cost of terephthalic acid. In this process, however, both the amount of solvent combusted and the total amount of catalyst used are greater than those in the prior art process in which a conventional catalyst is used. Therefore, in a commercial sense, there has still been room for improvement in the process just mentioned.
U.S. Pat. No. 4,051,178 issued on Sept. 27, 1977 discloses an improved catalyst for use in the preparation of terephthalic acid, which possesses higher activity and results in lower level of combustion of the solvent.
However, there is a need to provide a further improved process for preparing terephthalic acid of high purity at still lower cost.